Since you already disconected the hard drive and floppy with still no change, I would say it's most likely a problem with the power suppy or perhaps the mainboard itself.
You said the power led is flasing which might also indicate the 1200 isn't geting enough power.
You should first try converting your old ATX and see if that makes a difference.
If your in the UK and still in diffs PM me.. I have a spare power supply and mainboard you can have for free :-)
